ci: use a junction on Windows instead of a symlink

Symbolic links are still not quite as easy to use on Windows as on Linux
(for example, on versions older than Windows 10, only administrators can
create symlinks, and on Windows 10 you still need to be in developer
mode for regular users to have permission), but NTFS junctions can give
us a way out.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

ci/run-build-and-tests.sh

@@ -5,7 +5,10 @@
 
 . ${0%/*}/lib.sh
 
-ln -s "$cache_dir/.prove" t/.prove
+case "$CI_OS_NAME" in
+windows*) cmd //c mklink //j t\\.prove "$(cygpath -aw "$cache_dir/.prove")";;
+*) ln -s "$cache_dir/.prove" t/.prove;;
+esac
 
 make
 make --quiet test